P0171

Diagnostic Chart:

Circuit Description

In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control system uses a Closed Loop air/fuel metering system. While in Closed Loop, the PCM monitors the heated oxygen sensor signal voltage and adjusts fuel delivery based on the signal voltage. A change made to the fuel delivery will be indicated by long and short term fuel trim values which can be monitored with a scan tool. The ideal fuel trim values are around 0%. If the heated oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. A fuel trim DTC can set if exhaust emissions reach excessive levels because of a lean or rich condition.

Conditions for Running the DTC

^ Barometric pressure greater than 75 kPa.

^ Fuel tank level greater than 25%.

^ Intake air temperature is between -10°C (14°F) and 50°C (122°F).

Conditions for Setting the DTC

^ Long term and short term fuel trim greater than or equal to 30% for 5 seconds.

^ Short term fuel trim greater than or equal to 20% for 45 seconds.

Action Taken When the DTC Sets

^ The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.

^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

^ The PCM will enter the Fail-Safe Function and stop EGR and EVAP purge control.

Conditions for Clearing the MIL/DTC

^ The MIL will turn OFF after three consecutively passing cycles without a fault present.

^ A History DTC will clear after 40 consecutive warm-up cycles without a fault.

^ DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.

Diagnostic Aids

Check for any of the following conditions:

^ Check for vacuum leaks.

^ Check for exhaust gas leaks.

^ Check for the correct fuel pressure. The fuel system will be lean if fuel pressure is too low. It may be necessary to monitor the fuel pressure while driving the vehicle at various road speeds. Refer to Fuel System Diagnosis Table A1-7B. Table A1-7B Fuel System Diagnosis (Fuel Pressure Check)

^ Check for fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. Too much alcohol in the fuel can also cause a lean exhaust indication. Refer to Fuel Delivery and Air Induction / Testing and Inspection / Procedures for procedures to check for fuel contamination.

^ A Manifold Absolute Pressure (MAP) sensor output that causes the PCM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed (default) value for the MAP sensor. If the lean condition is gone when the MAP sensor is disconnected, substitute a known good sensor and recheck.

^ Check for a poor HO2S 1 or Mass Air Flow (MAF) sensor electrical connection at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal to wire connection.

^ Check for a damaged wiring harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving the connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

^ Check for a contaminated oxygen sensor. Silicone and other contaminants can cause the oxygen sensor to provide false lean exhaust readings. If contamination is found, determine the cause and correct the condition before replacing the HO2S.

After all the checks have been completed and a malfunction continues to occur, replace the PCM with a known good unit and retest.

If a DTC P0171 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

If any wiring repairs need to be made, Refer to Repair Procedures in Electrical Diagrams.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. If any DTCs other than Fuel Trim (P0171, P0172), Misfire (P0300 to P0303), and EGR (P0400) are set, diagnosis those DTCs first.

3. Review Freeze Frame data to determine when the DTC set. Always record this information.

4. Any one of these sensors could cause this DTC to set, check the performance of the MAP sensor, Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor and the Idle switch signal, repair any faulty conditions.

8. Before replacing the fuel injector check the resistance of the fuel injector: 0.5 to 1.5 ohms at 20°C (68°F).